The physiological mechanism that sets eye focus is directly related to the mechanism that sets convergence. If a three dimensional (3D) image is represented on a screen for a viewer, the parallax (or the difference between images that are sent to each of the viewer's eyes) must be set to make the object appear near to the screen so that the viewer's eyes can comfortably converge and focus on the screen at the same time. This makes the representation of objects that are some distance away from the screen uncomfortable for the viewer in conventional 3D systems because the point of focus is necessarily fixed on the screen. This is a particular problem for devices designed to be held close to the viewer, such as laptops, tablets, and mobile phones. With these devices, fixing the parallax such that objects appear to be in the plane of the display makes it difficult to represent real-life objects, such as people, which would normally be a long distance behind the screen. Furthermore, users of near screen devices with farsightedness or presbyopia must typically wear rear reading glasses or contact lenses in order to see the information displayed on the screen of devices held close to their eyes.